Richard Norman <rsnorman at mediaone.net> wrote in message =
news:zm_L6.48549$yd.332104 at typhoon.mw.mediaone.net...
> "Brian" <zhil at online.no> wrote in message
> news:AVVL6.7887$Ty6.170980 at news1.oke.nextra.no...> > "Marielle Fois" <im99_foa at nada.kth.se> skrev i melding
> > news:Pine.SOL.4.30.0105132056520.22994-100000 at my.nada.kth.se...> > > On Sun, 13 May 2001, Marielle Fois wrote:
> > >
> > > > In
> > > > addition, what is perceived by the left half of the retina of =
the
> > > > left eye and the right half of the retina of the right eye, is =
the
> > > > same.
> > >
> > > I am sorry, this is completely false.
> > >
> > > Marielle
> >
> > No, you were almost right.
> > The difference in both visual fields are calculated as the 'depth'.
> > Why it's devided on both eyes are to make sure that if you lose
> > one eye, you'll still have depth-vision.
> > Mvh
> > Brian
> >
>=20
> How much misinformation can we really tolerate here?
Misinformation to which is added teleological presumptions.
>=20
> The original post referred to the fact that most sensory
> and motor information pertaining to the left side of the
> body is dealt with by the right hemisphere and vice-versa
> in the vertebrate world.
>=20
> It then became a discussion of the details of the
> decussation at the optic chiasm, which is expressed
> incorrectly above. The temporal half of each retina
> is uncrossed, the nasal half is crossed (at least in
> animals like primates with primarily binocular vision
> and omitting the double representation of the fovea).
Which itself is less than double, just to be exact.
Though there is considerable binocularity of foveal
receptive fields, there are a large number of monocularly
responsive cells in V1, as has been known since=20
David Hubel and Torsten Weisel's studies, back in the late fifties/early =
sixties.
> This pattern of crossing ensures that the left visual
> field from whichever eye goes to the right side of
> the brain and the right visual field to the left side of
> the brain. An earlier poster correctly indicated that
> many animals with laterally placed eyes (little or
> no binocular overlap) show complete decussation.
Well, yes, I did as much. I'll note that your earlier
mention of the probable phylogenic constraints,
that resulted in decussations, consistent with current=20
ontogenic molecular findings, are another
pleasant spot of actual science amidst the folderol.
...Which apparently evoked no further interest, alas.
Now it has to do with cues for depth perception.
> Binocular vision deals with the fact that information
> from both eyes shows up on each side of the brain.
> The disparity in the two images produces very strong
> cues about depth. If you lose one eye you will lose
> all binocular disparity. However there are remain
> other cues for depth (which you can verify by closing
> one eye and looking around). =20
Best done by covering, rather than closing, as rotation
of the contralateral sphere is somewhat impeded by voluntary
closure-- though the angular displacement changes in the appearance
of solid objects and spatial volumes are still easily observable.
We could also mention depth cues of shadowing and spatial foreshortening =
which are available to monocular vision.
The fact that each eye
> is divided into nasal vs. temporal halves which separate
> at the chiasm is totally unrelated to making "sure that
> if you lose one eye, you'll still have depth-vision."
Teleological tee-hee, were it not that the poster was apparently
sincere in this notion.=20